Filter undefined from binding elements with initialisers without undefined in the type (microsoft#38122)

* Filter undefined from binding elts w/o undefined-containing inits

* use getTypeOfInitializer instead

* improve comment based on Wesleys suggestion

Author: sandersn

src/compiler/checker.ts

@@ -7305,8 +7305,8 @@ namespace ts {
             if (strictNullChecks && declaration.flags & NodeFlags.Ambient && isParameterDeclaration(declaration)) {
                 parentType = getNonNullableType(parentType);
             }
-            // Filter `undefined` from the type we check against if the parent has an initializer (which handles the `undefined` case implicitly)
-            else if (strictNullChecks && pattern.parent.initializer && isParameter(pattern.parent)) {
+            // Filter `undefined` from the type we check against if the parent has an initializer and that initializer is not possibly `undefined`
+            else if (strictNullChecks && pattern.parent.initializer && !(getTypeFacts(getTypeOfInitializer(pattern.parent.initializer)) & TypeFacts.EQUndefined)) {
                 parentType = getTypeWithFacts(parentType, TypeFacts.NEUndefined);
             }
 

tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.errors.txt

@@ -1,7 +1,8 @@
 tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts(7,9): error TS2339: Property 's' does not exist on type '{ s: string; } | undefined'.
+tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts(8,16): error TS2339: Property 's' does not exist on type '{ s: string; } | undefined'.
 
 
-==== tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts (1 errors) ====
+==== tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts (2 errors) ====
     const fInferred = ({ a = 0 } = {}) => a;
     // const fInferred: ({ a }?: { a?: number; }) => number
 
@@ -11,4 +12,7 @@ tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts(7,9
     const { s } = t;
             ~
 !!! error TS2339: Property 's' does not exist on type '{ s: string; } | undefined'.
+    function fst({ s } = t) { }
+                   ~
+!!! error TS2339: Property 's' does not exist on type '{ s: string; } | undefined'.
 

tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.js

@@ -6,6 +6,7 @@ const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
 
 declare var t: { s: string } | undefined;
 const { s } = t;
+function fst({ s } = t) { }
 
 
 //// [contextualTypeForInitalizedVariablesFiltersUndefined.js]
@@ -20,3 +21,6 @@ var fAnnotated = function (_a) {
     return a;
 };
 var s = t.s;
+function fst(_a) {
+    var s = (_a === void 0 ? t : _a).s;
+}

tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.symbols

@@ -20,3 +20,8 @@ const { s } = t;
 >s : Symbol(s, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 6, 7))
 >t : Symbol(t, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 5, 11))
 
+function fst({ s } = t) { }
+>fst : Symbol(fst, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 6, 16))
+>s : Symbol(s, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 7, 14))
+>t : Symbol(t, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 5, 11))
+

tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.types

@@ -26,3 +26,8 @@ const { s } = t;
 >s : any
 >t : { s: string; } | undefined
 
+function fst({ s } = t) { }
+>fst : ({ s }?: { s: string; } | undefined) => void
+>s : any
+>t : { s: string; } | undefined
+

tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts

@@ -6,3 +6,4 @@ const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
 
 declare var t: { s: string } | undefined;
 const { s } = t;
+function fst({ s } = t) { }